This invention relates to the cooling of thin walls for various aerospace applications and, more specifically, this invention relates to the cooling of thin walls using a waffle structure.
Various devices such as combustor liners, other exhaust structures or similar structures, and walls of rocket engine, need cooling. The air used for air/fuel mixtures can be used advantageously for that purpose. It is known that efficient burning of fuel is a goal for good turbine engine designs but the burning of fuel necessarily involves heat generation. Further, the burning of fuel also necessarily causes environmental problems necessitating the reduction of dangerous emissions. How to dissipate the generated heat and how to use the cooling air effectively becomes an important design issue. In addition, the difference in temperature in various parts of the thin walls challenges designers in that they need to consider both the efficiency of fuel burning and the structural integrity of the thin walls. Structural design of thin walls such as those for combustion liners is generally based on experience with previous systems. Various patents have addressed the issue.
U.S. Pat. No. 4,642,993, entitled Combustor Liner Wall, teaches a combustor liner wall with an interior wall, an exterior wall, and a honeycomb structure disposed therebetween. The honeycomb structure is formed with generally radially aligned cells. The patent teaches a structure known in the art as licolite. However, it does not address the issue of thermal growth differences between the hot and cold walls of a panel structure, which may result in high stresses for even moderately sized combustors. To solve the above problem, the walls are divided into small panels where spent cooling air is dumped. This may result in release of dangerous emissions.
U.S. Pat. No. 4,864,827, entitled Combustor, teaches a gas turbine engine combustor with a semi-spherical upstream wall comprised of two correspondingly shaped skins spaced apart by pedestals attached to one of the skins to define a space between them. In other words, this patent maintains a space between two walls by the use of pedestals. The two walls float, or move, relative to each other so as to relieve the geometrical changes that result from temperature variations. However, this does not allow a stable gap between the two walls. Further, for relatively large structures this patent may be impractical in that no stable gap can be maintained because the pedestals are not rigidly affixed to at least one of the walls.
U.S. Pat. No. 5,822,853, entitled Method for Making Cylindrical Structures with Cooling Channels, teaches a gas turbine having a double wall with a plurality of cooling channels therebetween. The cooling channels are formed between the inner member of the structure and the outer member thereof. In other words, the patent addresses a method of manufacturing, involving a sandwiched panel. However, the walls are not rigidly connected to each other and, therefore, the problem of thermal growth difference between the two walls is not resolved. Also, as can be appreciated, the fact that there is no rigid connection between the walls causes at least a reduction in efficiency with regard to heat transfer. Further, the lack of connection between the walls may make the concept impractical for larger structures.
As can be seen, there is a need for a method and apparatus to deal with thermal growth differences occurring between adjacently located hot and cold walls of panel structures, and which does not result in high stresses during operation. Further, it is desirable to have a simple structure that is easy to fabricate, and at the same time fulfills the need of cooling the apparatus. The waffle cooling satisfies that need. The second wall is an irregular non smooth surface that responds somewhat like a bellows, and expands and contracts with respect to the thermal expansion of the first surface thus reducing the stresses that are generated by the difference in temperatures between the two surfaces.
In one aspect of the present invention, there is disclosed an apparatus for cooling thin walls that comprises a first member, which has a smooth surface; a second member, which has an uneven surface disposed to form a plurality of cavities by rigidly affixing part of the uneven surface onto the smooth surface of the first member, and a plurality of openings connecting at least two cavities; and at least one opening in a first cavity for receiving incoming air, as well as at least one opening in a second cavity for disposing out-going air, whereby the temperature, as well as the temperature difference between the members, is reduced.
In another aspect of the present invention, there is disclosed an apparatus for cooling thin walls that comprises a first wall having a generally smooth shape with a first surface, and a second surface; a second wall having a waffle shape, wherein a plurality of cavities are formed, each cavity being defined by four sides with part of the cavity sides rigidly affixed to the first surface of the first wall; and a plurality of gaps formed at the sides of the cavities that are non-rigidly affixed to the first surface of the first wall, thereby forming a passage for air to flow from one end of the apparatus to the other through at least some cavities, whereby the first wall and the second wall are cooled.
In a further aspect of the present invention, a combustor is described comprising an inflow of air having a primary flow into an inner cylinder, and a secondary flow into an air casing defined and limited by the inner cylinder and an outer cylinder. The combustor further comprises a first wall that is part of the outer limit of the inner cylinder, which has a generally smooth shape, and which includes a first surface, and a second surface. A second wall is also provided that is part of the outer limit of the inner cylinder, which has a waffle shape within which there is a plurality of waffle shaped cavities. Further, each cavity is defined by four sides with part of the cavity sides being rigidly affixed to the first surface of the first wall, whereby a plurality of gaps is formed at the sides of the cavities that are non-rigidly affixed to the first surface of the first wall, thus forming a passage for air to flow from one end of the apparatus to the other end through at least some cavities. Thereby, the first wall and the second wall are cooled.
In yet another aspect of the present invention, a method for making a cooling device comprises the steps of providing a first member that has a generally smooth surface, providing a second member having a waffle shape with a plurality of trapezoidal shaped cavities each having four sides, and affixing at least some of the four sides onto the first member.